Health Library

Health Library

Anion Gap (Blood)

Does this test have other names?

Serum anion gap

What is this test?

This test looks at electrically charged particles in your blood to help your doctor diagnose acid-base imbalances. The test results are calculated from the results of an electrolyte panel, another blood test.

The value for the anion gap tells your doctor something about which charged particles besides sodium, chloride, and bicarbonate ions must be in your blood to make it neutral.

This test gives clues about different types of acidosis, when your blood is too acidic; and alkalosis, when your blood is not acidic enough. Acidosis in particular can be life-threatening, so it's important to find the cause and treat it as soon as possible.

Metabolic acidosis may be caused when:

Your body makes too much acid. For example, lactic acidosis may occur if you exercise too much. People with diabetes may develop ketoacidosis, in which their body makes acids called ketones. And certain poisons, such as methanol (wood alcohol), too much aspirin, or antifreeze, may also lead to acidosis.

You lose too much bicarbonate. This can happen if you have diarrhea. It can also happen with a condition called proximal renal tubular acidosis, in which the kidneys do not reabsorb enough bicarbonate and it is lost in urine.

Your body doesn't get rid of enough acid in your urine because of problems with your kidneys.

Why do I need this test?

You might need this test if you have symptoms of metabolic acidosis. Signs and symptoms include:

Fatigue

Shortness of breath

Nausea and vomiting

Rapid heart rate

Low blood pressure

What other tests might I have along with this test?

Your doctor may also order other blood tests because many conditions can affect the acid-base chemistry of your blood. Possible tests include:

pH levels

Dissolved gases such as carbon dioxide

Lactate and glucose

Creatinine

Blood urea nitrogen, or BUN

If you may have swallowed a poison, such as wood alcohol, salicylate (in aspirin), and ethylene glycol (in antifreeze), your doctor may test your blood for it.

If your doctor suspects you have ketoacidosis, an emergency complication of diabetes, you might need a urine dipstick test for ketone compounds.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Results are given in milliequivalents per liter (mEq/L). Normal results are 10 to 12 mEq/L, although the normal level may vary from lab to lab.

If your results are higher, it may mean that you have metabolic acidosis.

If you have hypoalbuminemia, or less albumin protein than normal, your expected normal result must be adjusted downward.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Being dehydrated or retaining water in your body can affect your results. Certain antibiotics, such as penicillin, can also affect your results.

How do I get ready for this test?

You don't need to prepare for this test. But be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.

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